Cylinder boring vs a lathe
by Richard Sheckler
Why would one prefer a lathe over a boring device made especially for boring cylinders? That, Duane, is a fine question, and I delight in your asking because I am about to tell you. 😉
A precision lathe like a Clausing can be set up for repeat accuracy using a shop-made jig to hold each cylinder, aligning each according to the cylinder bases and skirts. Before boring, the top of each cylinder can and should be checked for concentricity and alignment, one key objective in using a lathe. Another is guaranteeing that the centerline of the finished bore is perpendicular to the CL of the crankshaft. The third is to torque the cylinder base against the jig the same as the finished cylinder would against the crankcase to apply the same stress that occurs when the cylinder is in operation. This ensures the bore will be concentric when attached and torqued to the case.
I have received cylinders returned from prominent boring and cylinder services that had bores as much as .045″ off-center and 8 degrees away from perpendicular. Considering that these crankcases spread apart in the front over so many years, when the bore leans to the rear on such a case, the effect is cumulative, and a wrist pin failure is imminent. Worked on too many of these failures, and the bores were off the 90-degree mark every time.
Solution: Deck the crankcase (fly cut the mating surfaces for the cylinder bases on the case, equally on both sides) and bore the cylinders on a lathe.
My thanks to Richard for this great explanation. His explanation is one more example of how this site for BMW motorcycles got built by many contributors.
Updated 11 July 2022